what are you going to use the pc for? As usually, it depends on the applications.
Games still profit from single core performance, less cores but higher CPU frequency is preferred. If you are running multi-core optimized applications, then obviously more cores are needed.
If you are encoding video, or graphics rendering constantly, an 8 core or 6 core will benefit you. If you are gaming, or just generally
emailing, surfing the internet, and watching videos, a high clock dual core is adequate, and the money would be better spent , like a kickass video card, or upgrade to an ssd.
If you occasionally encode videos while using your pc, a quad core is probably a good buy.
If y aou buy a 6/8 core cpu and you aren't flogging it to death with video encoding or heavy rendering, then you are wasting money, and electricity.
6/8 chores cpus typically idle at full power load of a dual core cpu, of not higher, and possibly even waste enough to power a low end quad core at full tilt.